As a conventional speed reduction device of a vehicle, there has been known a speed reduction device which includes a driven sprocket gear, a cylindrical member which supports the driven sprocket gear, and a cylindrical housing which rotatably supports the cylindrical member. See, for example, JP-A-63-90486 (JP '486).
As shown in FIG. 1 of JP '486, the speed reduction device includes a cylindrical rear axle housing which is mounted on a rear frame of a vehicle body, a center rear axle which is rotatably mounted on the rear axle housing by way of ball bearings, a detachable flange which is integrally mounted on one end of the center rear axle, and a driven sprocket gear which is mounted on the detachable flange.
The center rear axle is a member which has an outer peripheral portion of another end thereof joined to a boss portion by spline engagement and has both sides of an inner peripheral portion thereof joined to respective input shafts of inner universal joints by spline engagement, wherein a brake disc is mounted on a boss portion, an outer universal joint is connected to each inner universal joint by way of a tiltable rear axle, and a support shaft is integrally mounted on the outer universal joint, and the support shaft and a hub on a rear wheel side are joined by spline engagement.
The speed reduction device adopts the structure which inserts and connects respective input shafts of inner universal joints from left and right into an inner peripheral portion of the center rear axle and hence, these inner universal joints largely project left and right with respect to the center of the vehicle body.
Accordingly, a distance between the inner universal joint and an outer universal joint of the rear wheel side which is positioned at a predetermined location in the vehicle width direction from the center of the vehicle body is shortened and hence, a tilting angle of a tiltable rear axle becomes large when the rear wheel is vertically moved whereby, for example, a moving quantity of balls positioned inside the inner universal joint and the outer universal joint is increased resulting in the large-sizing of the inner universal joint and the outer universal joint. Further, it becomes difficult to maintain frictions of balls in the inside of the inner universal joint and the outer universal joint at small values for a long period.
Accordingly, it is an object of the invention to maintain a friction at a connecting portion at a small value for a long period while reducing the size of connecting portions of drive shafts to left and right drive axles.